In recent years, a damper device has been used which is mechanically connected to a toilet seat or a toilet lid of a Western type toilet by taking into consideration of user's convenience when the toilet seat or the toilet lid is opened or closed.
In the damper device, a rotor shaft is disposed in the inside of a casing and oil (viscous fluid) is stored in a sealed space formed between the rotor shaft and the casing. The sealed space is formed by the casing incorporated with a cover after the rotor shaft is inserted into the inside of the casing and oil is stored between the rotor shaft and the casing (see, for example, Japanese Patent Laid-Open No. 2001-54490).
However, in a conventional damper device, the casing and the cover are fixed to each other with screws. Therefore, portions for fixing screws are protruded outward in a radial direction from the casing and the cover and thus the size of a damper device is enlarged in the radial direction. Further, screws are necessary for the fixing portions and thus the number of component parts increases.
In order to solve the above-mentioned problem, it is conceivable that the casing and the cover are fixed by ultrasonic welding instead of fixing with a screw. However, when ultrasonic welding is performed with the horn of an ultrasonic welding device abutting with the end face of the cover in an axial direction, the casing is crushed in the axial direction and the end face of the cover for forming the sealed space may be pushed. Therefore, a stable gap space is not obtained which is formed between the rotor shaft disposed in the inside of the casing and the end face of the cover. In the worst case, the rotor shaft and the end face of the cover abut with each other to cause malfunction of the rotor shaft.